(1) Field of the Invention
This invention relates to a method for increasing the permeability of a petroleum-containing subterranean reservoir penetrated by a well. More particularly, the invention relates to a method of acidizing or fracturing such a reservoir with reduced damage to the reservoir by the displacement fluid used to displace the acidizing or fracturing fluid into the reservoir and by the fluid used to kill the well.
(2) Description of the Prior Art
Many wells are completed in relatively low permeability petroleum-containing reservoirs made up at least partially of acid-soluble components. In order to increase the permeability of the reservoir surrounding such wells, which permeability can be further decreased by having come in contact with drilling fluids during the drilling operation, so that petroleum can be more easily produced therefrom or an enhanced recovery fluid can be more easily injected therein, it is common practice to acidize and/or acid fracture the reservoir. In such processes, an acid solution is injected down a well conduit and out into the reservoir. After the desired amount of acid solution has been introduced into the well, it is followed by a slug of displacement fluid which displaces the last portion of acid solution remaining in the well conduit out into the reservoir. Since the reservoir can contain fluids at a high pressure, it is usually necessary to follow the slug of displacement fluid with a weighted fluid to occupy the well conduit and balance the reservoir pressure. Such a procedure is referred to as killing the well. When the well is in this condition, it is common practice to shut it in for a period of time to enable it to be prepared either for production of well fluids or for injection of an enhanced recovery displacement fluid. This preparation can include running tubing into the well, setting a packer, installing a christmas tree and other surface fluid handling equipment and the like. During these preparations, there is a tendency for the column of displacement fluid and kill fluid in the well to leak away into the reservoir since the weight of the column of fluid in the well conduit is greater than the reservoir pressure. Thus, the voids in the newly acidized and/or fractured reservoir are invaded by displacement fluid and kill fluid. When the well is subsequently put on production or injection, this displacement fluid and kill fluid can cause at least some blockage and plugging of the reservoir permeability and at least partially offset the permeability-increasing effects of the acidizing/fracturing operation. This is especially true in the case of the kill fluid which is generally a weighted aqueous drilling mud composition. This drilling mud exhibits considerable gel strength. While the well is shut in, the drilling mud becomes stagnate, tends to gel, can loose moisture and form a mass of partially dry solids, and becomes quite resistant to subsequent movement when the well is put in service.
Similar problems of reservoir permeability damage caused by invasion of a reservoir by displacement fluid and kill fluid exist where wells are hydraulically fractured using nonacid fracturing fluids such as fresh water, brine or oil.
It is known to restrict the flow of fluid from a wellbore into a reservoir for a period of time during various well operations by injecting into the well a ball or plug which lodges downhole in a seat, baffle or perforation and either stops further fluid flow through the wellbore or diverts the flow to an alternate route. The ball or plug can be subsequently removed. U.S. Pat. No. 3,547,197 to Chevalier et al. describes a method of acidizing a perforated well comprising:
(a) injecting a first slug of an acidizing solution, PA1 (b) injecting a slug of water to displace the acidizing solution out into the reservoir, PA1 (c) injecting a slug of water containing a number of sealer balls to lodge in and seal the more permeable perforations, and PA1 (d) injecting a second slug of the acidizing solution which will invade the less permeable portions of the reservoir. PA1 (a) running into the well a tubing string with a baffle placed in the string at a position between the upper and lower formations, PA1 (b) cementing the tubing in place, PA1 (c) perforating and fracturing the lower formation, PA1 (d) pumping down the tubing a ball or plug which lodges in the baffle, PA1 (e) perforating and fracturing the upper formation, and PA1 (f) producing the well, thus, floating the ball out of the tubing.
U.S. Pat. No. 3,289,762 to Schell et al. shows a method of fracturing both an upper formation and a lower formation penetrated by a well comprising:
U.S. Pat. No. 2,796,130 to Huber discloses an apparatus for depositing in and removing from a well containing casing and tubing a cement slurry or a gelled acid treating agent. A valve seat is positioned in an extension conduit below the tubing. As part of the operation of the apparatus, a ball is dropped through the tubing to seat in the valve seat after the cement slurry or gelled acid is injected. The extension conduit then drops down, as from above well perforations to below the perforations. Excess cement slurry or gelled acid treating agent is then reverse circulated out of the annulus surrounding the extension conduit. After the treatment is complete, the extension conduit and ball are pulled from the well by a wire line to open up the tubing.
Thus, while various well treatments are known in which balls are dropped to temporarily stop the flow of fluid down a wellbore, it has not been previously known to employ such a technique during a well acidizing or fracturing operation in a manner so as to prevent the displacement fluid and kill fluid injected following the acidizing or fracturing fluid from invading and adversely reducing the reservoir permeability.
Accordingly, a principal object of this invention is to provide a method for acidizing and/or fracturing a hydrocarbon-containing subterranean reservoir.
A further object is to provide such a method wherein contamination of the so-treated reservoir by the fluid used to displace the acidizing or fracturing fluid out of the wellbore and into the reservoir and the kill fluid is reduced.
A still further object is to provide such a method wherein the permeability of the so-treated reservoir is not decreased by invasion of displacement fluid and kill fluid.
Another object is to provide such a method wherein the reservoir permeability is not decreased during subsequent preparation of the well for production or injection.
Yet another object is to provide a method for killing a well following an acidizing or fracturing treatment.
Other objects, advantages and features will be apparent from the following description and drawings.